1. Field of the Invention
This invention relates to a developing device, and more particularly, it is concerned with a developing device which visualizes a latent image by use of a single component magnetic developer.
2. Description of Prior Arts
For the dry developing method for developing an electrostatic pattern, there are two types from the standpoint of construction of the developing methods, i.e., two-component development method and one-component developing method. The former method uses a developing agent consisting of carrier particles such as iron powder, glass beads, etc., and toner particles which actually develop an electrostatic image. This two-component developing method has the disadvantage of downgraded image quality due to variations in the image density caused by changes in the mixing ratio between the carrier particles and the toner particles, and deterioration in the carrier particles. On the other hand, the single-component developing method is free from the deficiency of the two-component developing method, because of its absence of the carrier particles. Therefore, this latter method has high potential as the developing method of the future.
The one-component developing agent which has generally been known and used includes magnetic powder in the toner particles due to the need for providing various means such as, for example, the property of conveying the developing agent upto the developing region opposite to the electrostatic image, and the property of frictional charging by relative movement. The content of the magnetic powder is nevertheless naturally limited, because the toner is bonded onto an image transfer paper using heat, pressure, or other means so as to fix the toner image on the image transfer paper to it. Practically, the magnetic powder occupies 10 to 60% by weight of the toner particles. However, owing to the difference in the specific gravity between the resin constituting the toner and the magnetic powder, the volumetric occupying ratio of the magnetic powder in the toner particles is 20% or less. Since the volumetric occupying ratio of the magnetic powder in the toner is very low, behavior of the toner in the magnetic field, unlike the single body of the magnetic powder, makes it difficult to form a long brush of scattered density at the position of the magnetic pole. On account of this, if the thickness of the toner layer on the toner holding member is restricted to an order of a few millimeters, there tends to occur irregularity in the toner layer formed on the toner holding member, hence non-uniformity in its thickness. This non-uniformity of the toner layer on the toner holding member tends to directly reflect on the image as developed. Further, since the toner layer is generally dense, when irregularity occurs in its layer thickness, there would take place such dangers that the toner particles are coagulated by press-contact of the particles to the surface of a photo-conductive body which is the electrostatic image bearing section, or the photo-electric body itself is impaired. In this context, therefore, the developing method which utilizes the one-component magnetic toner is required to form a thin uniform layer of the toner on the toner bearing member. In general, with a view to regulating the layer thickness of the powder material on the toner holding member, when the thickness regulating member is disposed in contiguity to the surface of the holding member to form a slit, and the holding member performs its relative movement with respect to the thickness regulating member, the toner layer thickness which has been subjected to actual thickness control will be slightly thicker than the abovementioned slit interval. Under such circumstances, in the development using the one-component magnetic toner, the thickness regulating member has had to be brought very close to the toner bearing member so as to form the thin layer of the toner. On account of this, mechanical precision has been required of the developing device, and, moreover, there has taken place such situations that the toner particles which have been coagulated for various reasons come into small gap of the slit causing no toner layer formation on that specific portion. Furthermore, in spite of the fact that the developing device using the one-component magnetic toner is of a simple mechanical construction, the conventional developing devices possess some disadvantages of making it difficult to form a thin, uniform toner layer. In more detail, when the toner thickness regulating member is press-contacted to the toner layer, if the pressure is small, dropping and scattering of the toner particles increase, and, if the pressure is great, the toner particles coagulate among themselves, or they stick to the thickness regulating member or the conveying and holding member. Such coagulation and adhesion cannot be avoided in a method, in which the toner particles are caused to pass through a given gap. Such scattering, coagulation, and adhesion of the toner particles remarkably deteriorate the image as developed. Moreover, when an additional device is provided to solve the afore-described disadvantages, the advantage of the device, which is simple in construction due to use of the one-component magnetic toner, cannot be made much use of.
U.S. Pat. No. 4,081,571 and German Laid-open patent application Ser. No. 2810520 disclose a developing device having a sleeve and a doctor blade to determine thickness of a magnetic brush, wherein an element made of a magnetic material is fixedly provided on one part of the doctor blade.
Laid-open Japanese Patent Application No. 53-125844 describes a developing device of a type, wherein electrically conductive magnetic toner is adhered onto an outer peripheral surface of a fixed sleeve having a rotating magnet roll by magnetic force of the magnet roll, and the toner particles are conveyed by rotating the magnet roll to carry out the development, the layer thickness of the toner adhered onto the outer peripheral surface of the sleeve being regulated uniformly by means of magnetic force generated from the magnet roll and the magnetic body.
Further, a co-pending U.S. patent application Ser. No. 938,494, filed on Aug. 31, 1978, now abandoned, of the same assignee as that of the present application discloses an art of using one-component magnetic toner and a magnetic doctor blade and magnetic field for the uniform control of the toner layer on the sleeve.
These patents and patent application do not disclose anything about the point of improvement which is the subject matter of the present invention and described in the ensuing preferred embodiments.